Selenium (Se) isotopes have been widely used to track the Earth's redox evolution based on the assumption that terrestrial Se is faithfully transported at the land-ocean interface without nonconservative behavior or biological fractionation affecting its isotope composition. However, dissolved Se isotope behavior in estuaries is poorly constrained, which calls this assumption into question. In this study, we present the first dissolved inorganic Se (DISe) isotope dataset along a salinity gradient during transport of the Changjiang river. Water and phytoplankton samples from a bloom event were collected. The δ^82/76DISe values increased from 0.96‰ to 1.74‰ along the salinity gradient (from 0.1 to 34.5). The nonconservative behavior of DISe was revealed by large offsets of δ^82/76DISe from theoretical mixing of the bloom waters. The dynamics of δ^82/76DISe in the Changjiang Estuary (CJE) were controlled by the combination of physical mixing and biological utilization. In those bloom waters, the heavy δ^82/76DISe values (1.66‰ ± 0.07‰) in the waters corresponded to the light Se isotopes for phytoplankton (−0.35‰ ± 0.18‰). The significantly increased δ^82/76DISe signatures in the bloom waters due to the utilization of DISe by phytoplankton led to an Se isotope fractionation factor (reported as ε^82/76Se = 1000 × (α^82/76Se − 1) in) of −1.92‰ ± 0.16‰ (Rayleigh model) or −2.14‰ ± 0.18‰ (steady-state model). We propose that biological activities modify the nonconservative activity of Se and induce significant Se isotope fractionation with a factor of approximately −2‰ during transport. Our findings highlight that interpreting Se isotopes in the sedimentary record requires the consideration of nonconservative behavior and biological fractionation of Se.

硒 (Se) 同位素已被广泛用于跟踪地球的氧化还原演化，这是基于陆地硒在陆地-海洋界面忠实传输而不会影响其同位素组成的非保守行为或生物分馏这一假设。然而，河口中溶解的硒同位素行为受到的限制很差，这使这一假设受到质疑。在这项研究中，我们展示了长江运输过程中沿盐度梯度的第一个溶解无机硒 (DISe) 同位素数据集。收集了来自开花事件的水和浮游植物样本。沿盐度梯度（从0.1到34.5） ，δ ^82/76 DISe值从0.96‰增加到1.74‰。^{δ 82/76}的大偏移揭示了 DISe 的非保守行为DISe 来自开花水的理论混合。长江口（CJE） δ ^82/76 DISe的动态受物理混合和生物利用相结合的控制。在这些水华中，水中重的 δ ^82/76 DISe 值（1.66‰ ± 0.07‰）对应于浮游植物的轻硒同位素（-0.35‰ ± 0.18‰）。由于浮游植物对 DISe 的利用，在水华中显着增加的 δ ^82/76 DISe 特征导致 Se 同位素分馏因子（报告为 ε ^82/76 Se = 1000 × (α ^82/76Se - 1) in) 为 -1.92‰ ± 0.16‰（瑞利模型）或 -2.14‰ ± 0.18‰（稳态模型）。我们建议生物活性改变硒的非保守活性并在运输过程中诱导显着的硒同位素分馏，其系数约为-2‰。我们的研究结果强调，解释沉积记录中的硒同位素需要考虑硒的非保守行为和生物分馏。